1. Field of the Invention
The present invention relates to a steel which is used as a material for gears such as for a gear used in an automatic transmission, and relates to gears superior in strength of a tooth surface.
2. Description of the Prior Art
Conventionally, almost all typical gears have been made of chromium steel or chrome-molybdenum steel, such as JIS (Japan Industrial Standard) SCr 420H or JIS SCM 420H. Such gears have been treated by carburizing-hardening-tempering after forming in a gear-shape. Since the gears for automatic transmissions are required to have high strength on tooth surface so as to be durable to repeated friction under high contact pressure such as more than 2000 MPa, it have been necessary to apply some special treatments in the tooth surface, for example, a high-density carburizing method which strengthens the tooth surface by precipitating micro carbide in a surface layer, or a solid lubrication method which decreases the friction on the tooth surface by forming a solid lubrication film on the surface. However, these methods invite some problems such that it is necessary to take a long time for applying such treatments to the gears and therefore production cost is increased. On the other hand, although pitting resistance and wear resistance of the material are improved by increasing the amount of alloying elements, the hardness of the materiel is simultaneously increased. Therefore, the forgeability of the material is degraded. This shortens lives of machining tools for gear forming.